Conversion of amplitude modulation to frequency modulation



June 19, 19457` w. VAN B. ROBERTS 2,378,581

CONVERSION OF AMPLITUDE MODULATION TO FREQUENCY MODULATION Filed June25, 1942 F/FEQ. //MZ') (uffa/vrs Hike' 7 05cm LA Tof? v INVENTOR Muff?K1N iosa-frs.

A'FTRNEY Patented June 19, 1945 CONVERSION F AIVIPLITUDE MODULATION j T0FREQUENCY MODULATION walter van Roberts, Princeton, N. J., signor toRadio Corporation of America, a corporation pf Delaware Application June25, 1942, serial No. 448,343 9 claims. (ol. 25o-15) This inventionrelates to a method of and apparatus for directly converting anamplitude modulated radio frequency wave into a frequency modulatedradio frequency wave without, the need for an intervening detectionstep. In the past, it has been customary in converting an amplitudemodulated radio frequency wave to a frequency modulated frequency waveto employ suitable detecting apparatus to convert the amplitudemodulated wave to audio frequency and then to employ this audiofrequency to modulate the frequency of a frequency modulationtransmitter. This process introduces a certain amount of distortion,commonly called detector distortion, in the detecting means.

It is an object of the present invention to avoid such detectordistortion by providing means for converting amplitude modulated wavesto frequency modulated waves withoutvgoing through the processofdetection.

In brief, this object is achieved by employing as a frequencydetermining element in an oscillator generating the oscillations to befrequency modulated, an inductance Whose magnitude varies in accordancewith the amplitude of the amplitude modulated waves. This action isbased upon the following experimentally determined phenomenon. If aninductance coil is wound upon a suitable iron core, and if themagnetization of this core is caused to vary at a high frequency, thenthe effective inductance of the coil measured at a relatively lowfrequency is found to increase as the amplitude of the aforesaidvariation of magnetization increases. Specifically, it has been foundthat if the variation of magnetization of the core is produced by anauxiliary winding Carrying the high frequency current, the effective lowfrequency inductance is found to be substantially proportional to thesquare of the amplitude of the high frequency magnetizing current over awide range of amplitude. For a more detailed description of thisphenomenon, reference is herein made to copending Gerard Heppapplication, Serial No. 334,530, led May 11, 1940, now United StatesPatent 2,291,715,

granted August 4, 1942.

A more detailed description of the invention follows in conjunction witha drawing, wherein Figs. 1 and 2 show, by way of example only, twodiiferent radio relaying systems embodying the principles of the presentinvention, for converting an amplitude modulated radio frequency Waveinto a frequency modulated radio frequency wave. Throughout the twongures. the same parts are labeled by the same referenceV characters.

Although the two figures of the drawing illustrate the principles of theinvention applied to a radio relaying system, and particularly to a,repeater station, it should be understood that the invention has widerapplication.

Referring to the radio repeater station of Fig. 1, it is assumed thatamplitude modulated radio frequency waves are received by an antenna l.These received waves are amplied and/or changed in frequency inapparatus 2, and then passed over leads I I to magnetizing coils 3 and 4which are wound on iron cores 5 and 6. Coils 'l and 8 are also Wound oncores Sand 6, respec- `tively, and these last coils form the inductanceor at least a. portion thereof of the tank circuit 9 of an oscillator,system indicated by the legend oscillator. The exact type of oscillatorcircuit used is immaterial butthe frequency of oscillation is chosen lowcompared to the frequency of the amplitude modulated current traversingthe magnetizing windings 3f and 4. The oscillator frequency may,conversely, be as high as desired, if a suitable increase of thefrequency of the amplitude modulated waves to satisfy the aforesaidrequirement is provided. 'I'he oscillator frequency is preferably ofradio frequency, but could even be an audio frequency oscillatoralthough in such case a considerable number of stages of frequencymultiplication would be required to raise the frequency of the frequencymodulated waves to the high frequencies commonly used for theirtransmission over the ether.

The operation of the arrangementl is as follows: 'I'he coils 3 and 4 arepreferably connected in series so as to assure equal currents in each ofthem, and the relative polarity of connections is so chosen (asindicated in the drawing) that there is no nefl high frequency voltageinduced in tank circuit 9 by the high frequency current in coils 3 and4. It is not essential that separate cores 5 and 6 and separate coilsb'employed, as the same results may be obtained to a somewhat lesserdegree (as shown in- Fig. 2) by the use of a single coil 1 on a singlecore 5 in tank circuit (to replace the coils l and 8) netizing coil 3'(to replace coils 3 and 4) having the amplitude modulated radiofrequency currents flowing both ways through this coil from .themid-tap. In fact, it is not necessary to avoid the induction of highfrequency currents in the oscillator tank, but it is preferable simplyto avoid the waste of highl frequency power. In any case, the eectiveinductance in the tank circuit, measand a mid-tapped magured at thenatural frequency of the tank circuit, is substantially proportional tothe square of the amplitude of the magnetizing high frequency current sothat the natural period of the tank circuit 9 which isproportional tothe square root of the effective inductance will be directlyproportional to the amplitude of the high frequency magnetizing current.If the natural period of the oscillator is proportional to the amplitudeof the high frequency current, then for small percentage changes in thisnatural period the variation of frequency of the oscillator will besubstantially linearly related to the variation of amplitude to the highfrequency exciting currents, and thus the amplitude modulation of theexciting current is converted substantially without distortion intofrequency modulation of the oscillations generated in tank circuit 9.These frequency modulated oscillations may then be amplified and/ormultiplied in frequency in electron discharge device apparatus IZ andfurther multiplied in frequency, amplified, limited, and converted infrequency in apparatus I3, in accordance with well known practice, toobtain any desired output frequency to be radiated from antenna ltogether with any desired amount of maximum frequency deviationcorresponding to 100% modulation of the amplitude modulation current.

Although the invention has been described in connection with receivingamplitude modulated waves and rebroadcasting them in the form offrequency modulated waves, it will be appreciated that the system may beemployed as a unitary means for generating frequency modulated Wavesfrom audio signals if a small amplitude modulation transmitter is usedto generate the magnetizing current referred to.

What is claimed is:

1. The method of varying the frequency of a low frequency oscillatorincluding as a frequency determining element thereof an inductor woundupon an iron core, which comprises magnetizing said core in accordancewith the amplitude of a relatively high frequency amplitude modulatedradio frequency Wave.

2. The method of varying the frequency of a low frequency oscillatorincluding as a frequency determining element thereof an inductor Woundupon an iron core, which comprises magnetizing said core in accordancewith the amplitude of a relatively high frequency amplitude modulatedradio frequency wave, and multiplying the frequency of oscillationsproduced by said oscillator.

3. In combination, an oscillation generator, a frequency determiningelement for said generator comprising an inductance coil Wound upon amagnetizable core, and another coil Wound on s aid core, and means forsupplying said last coil with amplitude modulated alternating currentsof a frequency higher than the frequency of oscillation of saidgenerator, whereby the magnetization of said core is varied inaccordance with the amplitude of said alternating currents, said means'being so connected to said last coil that no voltage is induced in saidinductance coil by tsaid amplitude modulated alternating curren s.

4. An amplitude modulation to frequency mod,

of amplitude modulated alternating currents of a frequency higher thanthe frequency of oscillation of said generator coupled to said coil insuch manner that the magnetization of said core is varied in accordancewith the amplitude of said alternating current but no voltage is inducedin said inductor element by said amplitude modulated currents, and meanscoupled to said converter for utilizing the frequency modulated Wavesproduced in said converter.

5. An amplitude modulation to frequency modulation converter comprisinga radio frequency oscillation generator, a frequency determining circuitfor said generator including as part thereof an inductor element woundupon a magnetizable core, a coil for magnetizing said core, and a sourceof amplitude modulated radio frequency currents of a frequency higherthan the frequency of oscillation of said generator coupled to saidcoil, whereby the magnetization of said core is varied in accordancewith the amplitude of said amplitude modulated currents.

6. An amplitude modulation to frequency modulation converter comprisingan oscillation generator, an inductor element whose value determines inpart at least the frequency of oscillation of said generator, saidelement being wound upon a magnetizable core, a coil for magnetizingsaid core Wound upon said core, and a source of amplitude modulatedalternating currents of a frequency higher than the frequency ofoscillation of said generator coupled to said coil.

'7. An amplitude to -frequency modulation converter comprising anoscillation generator having a parallel tuned circuit of a condenser andan inductance coil controlling the frequency of the oscillationsproduced by said generator, said inductance coil being wound upon amagnetizable core, another coil wound upon said core for magnetizingsaid core, a connection between both terminals of said last coil, asource of amplitude modulated alternating current of a frequency higherthan the frequency of oscillation of said generator, and a pair ofconnections extending from one terminal and from the mid point of saidlast coil to said source, whereby the magnetization of said core isvaried in accordance with the amplitude of said alternating current butno voltage is induced in said inductance coil by said amplitudemodulated current, and means coupled to said generator for utilizing thefrequency modulated waves produced in said converter.

8. A radio relaying station comprising a receiving antenna forcollecting amplitude modulated Waves radiated from a distant station, anoscillation generator, an inductor element Whose Value determines inpart at least the frequency of oscillation of said generator, saidelement Ibeing wound upon a magnetizable core, a coil for magnetizingsaid core wound upon said core, means for supplying said coil withamplitude modulated currents representative of the modulation on theWaves collected by said antenna but of a frequency higher than thefrequency of oscillation of said generator, said coil being coupled tosaid means in such manner that the magnetization of said core is variedin accordance With the amplitude of said currents supplied to said coil,and a transmitting antenna coupled to said oscillation generator.

9. An amplitude to frequency modulation converter comprising anoscillation generator having a parallel tuned circuit controlling thefrequency of the oscillations produced by said generator,

said tuned circuit including a pair of coils in series relation, eachWound upon a magnetizable core,

other coils wound upon said cores, one for each core, means connectingsaidi'last coils in series relation and to a source of amplitudemodulated alternating currents of a frequency higher than the frequencyof oscillation of said generator, the

